Exposure control system for photographic cameras



Aug. 19, 1969 TAKAYOSHI sATo ETAL 3,461,786

mxrosunn common SYSTEM FOR PHOTOGRAPHIC cmmms Filed 001;. 25, 1966 2 Sheets-Sheet 1 mvz'n'rons.

TAKAYOSHI SATO, HIROSHI UEDA,

ATSUTADA NAKATANI, Minoan OTAKE,

ATTORNEY.

TAKAYOSHI SATO ET AL 3,461,786

' EXPOSURE common SYSTEM FOR PHOTOGRAPHIC CAMERAS Filed Oct. 25. 1966 2 Sheets-Sheet 2 FIG. 3

FIG. 4

SW2 INVENTORS.

TAKA YOSHI SATOJ'IIROSHX UEDA,

ATSUTADA NAKATANI, MINORU OTAKE. BY W 1107 ATTORNEY,

Unitcd States Patent 3,461,786 EXPOSURE CONTROL SYSTEM FOR IHOTOGRAPHIC CAMERAS Takayoshi Sato, Hiroshi Ueda, Atsutada Nakatani, and Minoru Otake, Tokyo-to, Japan, assignors to Kabushiki Kaisha Koparu, Itabashi-lku, Tokyo-to, Japan Filed Oct. 25, 1966, Ser. No. 589,297 Claims priority, application Japan, Nov. 6, 1965, 40/68,172; May 27, 1966, ll/34,128; July 12, 1966, 41/ 15,540; July 13, 1966, 41/66,."140; July 14, 1966, ail/66,671; July 19, 1966, t1/68,372 Int. Cl. G031) 7/08, 9/02 US. Cl. 95--64 9 Claims ABSTRACT OF THE DISCLOSURE An exposure control system for use in photographic cameras wherein the diaphragm is operated by an electromagnetic device including a permanent magnet and a movable element wound with an energizing coil and a damping coil. The electromagnetic device is controlled by a Wheatstone type electric bridge having a photoresistor in one of the branches. Switching transistors of the p-n-p and n-p-n type are connected between the output terminals of the bridge, and a transistor amplifier is connected to the switching transistors. The diaphragm is controlled in quick and accurate response to changes in brightness of the object to be photographed by connecting the energizing coil between the output terminals of the transistor amplifier and by connecting the damping coil between the output points of the bridge.

BACKGROUND OF THE INVENTION Field of the invention The present invention relates to an exposure control system for use in photographic cameras, and more particularly to a system for automatically controlling a diaphragm device used in the photographic cameras which may be either still cameras or motion picture cameras.

Description of the prior art A known exposure control system for use in photographic cameras comprises a bridge circuit of a Wheatstone type having, in one of the branches thereof, a photoconductive element which undergoes a change in its resistance value in response to the change in the intensity of the incident light and which serves as a detecting element to convert the magnitude of the change in the light transmitted from the object to be photographed to the film into a change in the resistance value of said photoconductive element; a switching circuit comprising a pn-p type transistor and an n-p-n type transistor and being controlled by the voltage generated between the output terminals of the Wheatstone bridge when said bridge is rendered to an unbalanced state due to said change in the resistance value of said photoconductive element; an electromagnetic device (for example, electric motor or electric meter) comprising an energizing coil member inserted for connection between the common emitter electrode of said p-n-p type transistor and said n-p-n type transistor and the terminals of the respective power sources connected to the respective collector electrodes of said transistors, and at least a permanent magnet which interacts with said energizing coil member; said exposure control system being operative in such manner that said energizing coil member is displaced by a current in the energizing coil of said energizing coil member in a direction complying with the polarity of the voltage generated between the output terminals of said Wheatstone bridge to actuate the 3,461,786 Patented Aug. 19, 1969 diaphragm device in synchronism with the displacement of said coil member and to also maintain the amount of light transmitted from the object to be photographed to the film constant by controlling the Wheatstone bridge so as to be rendered to a balanced state again.

In the known exposure control system described above, the amount of light transmitted to the film is not always precisely controlled on account of the occurrence of oscillation of the movable element including said energizing coil member back and forth about the equilibrium point of said bridge or on account of the occurrence of stepwise displacement of said movable element during its displacement motion which result: from the response characteristic of the photoconductive element represented by a change in its resistance value corresponding to the change in the amount of the incident light, or in other words, the time delay which takes place in the photoconductive element in effecting a change in its resistance value in response to the change in the intensity of the incident light thereto; from the performance characteristics of the switching transistors; from the state of magnetic coupling between the electric control circuit and the electromagnetic device; from the inertia of the movable element including said energizing coil member; and from a failure in the actuation of said switching transistors in case the voltage generated between the output terminals of said Wheatstone bridge is small.

SUMMARY OF THE INVENTION It is, therefore, the primary object of the present Invention to provide an exposure control system of the type described, comprising switching transistors, a movable element and a damping coil member wound around said movable element and being operative so that the polarity of the voltage induced in said movable element due to its displacement is reversed with respect to the polarity of the voltage generated between the output terminals of the Wheatstone bridge, said damping coil member being inserted between the common base electrode and the common emitter electrode of the switching transistors, to thereby cause a constant current to flow in the energizing coil member to insure that said movable element always eifects movements at a constant speed and that said movable element instantaneously stops its 1 movement upon said bridge being rendered to a balanced state, whereby a correct diaphragm aperture may always be determined.

Another object of the present invention is to provide an exposure control system which is adapted to change the diaphragm aperture into a precise size in response to a slight change in the brightness of the object to be photographed, by preventing the deterioration of the performance sensitivity due to the influence of the base voltage of the switching transistors and by immediately displacing the movable element upon the occurrence of a slight voltage between the output terminals of the Wheatstone bridge.

Still another object of the present invention is to provide an exposure control system which is adapted to work so that the diaphragm aperture is smoothly and continuously altered, by eliminating the alternating current of a relatively high frequency which is apt to be caused to flow in the energizing coil member and thereby effecting a smooth and continuous displacement of said movable element.

A further object of the present invention is to provide an exposure control system which is suitable for a TTL (through the lens) system.

The present invention and other objects thereof will become more clearly understood by reading the following detailed descriptions with reference to the accompanying drawings which are given simply by way of examples.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a somewhat schematic representation of an embodiment of the present invention equipped with the exposure control means of the present invention;

FIG. 2 is a front elevational view, in an enlarged scale, taken along the line IIII in FIG. 2;

FIG. 3 is a side elevational view, with part broken away, of the exposure control means in FIG. 2; and

FIG. 4 is a wiring diagram of the electric control circuit which is connected to the exposure control means of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1 of the drawings, an adjustable focusing lens group 2 is mounted to the front wall In of the camera housing 1.

In the background of said lens group 2 is disposed a half prism 5 for introducing a part of the light reflected from the object to be photographed into the optical system 3 used for a range finder.

An eyepiece 4 of the range finder is adjustably mounted to an upper portion of the rear wall of the camera housing. In the background of said half prism 5 and on the lens axis 0 of the focusing lens group 2 are arranged, successively on one line, a diaphragm device 7 which is operated by an electromagnetic device 6, a master lens group 8, and a half mirror 9. The light reflected by said half mirror 9 is adapted to enter into a photoresistor element 10 comprising a photoconductive material such as cadmium sulfide, while the main light which has passed through said half mirror 9, is adapted to be transmitted to the surface of a film 12 through a suitable shutter device 11.

Though not shown, the shutter device 11 and the film 12 are driven synchronously by a known means. Said electromagnetic device 6 and said photoresistor element 10 are connected to said electric control circuit as will be described later.

Referring next to FIGS. 2 and 3, the structure of the exposure control means including said electromagnetic device 6, said diaphragm device 7, a master lens group 8, a half mirror 9 and a photoresistor element 10 will be described in detail.

The aforesaid electromagnetic device 6 is mounted to the back of a base plate 13 which is detachably mounted, by means of screws 50 or the like (not shown), to the front part of the camera housing 1 which is partly shown with dot-dash line 1b. This electromagnetic device 6 has a casing 14 of a cylindrical shape. A cylindrical magnet 15 is fixed to the internal peripheral surface of said casing 14.

A cylindrical movable element 16 having a main shaft 16a which is rotatably supported by the side walls of said casing 14 is inserted inside said cylindrical magnet 15 in concentric relationship therewith. The cylindrical movable element 16 is wound with an energizing coil L and also with a damping coil L which are connected to the electric control circuit as will be described later independently of each other, and is so arranged as to be operative in such a manner that when a current is caused to flow in said energizing coil L said cylindrical movable element 16 is adapted to be angularly displaced by virtue of the interaction between the magnetic field produced by said energizing coil L and the magnetic field of said cylindrical magnet 15. One end of said main shaft 16a extends through the base plate 13 and the front face thereof. A control disk 17 which is restricted in its range of movement by a pin 13a protruding outwardly from the face of the base plate 13 and which is provided with a slot 17a, is fixed to said one end of main shaft 16a.

Also, on the back of said base plate 13 is securely mounted, adjacent to said electromagnetic device 6, a box 18 by means of a flange 18a and screws. The end wall of said box 18 located on the side having flange 18a is open, while the opposing end wall is provided with an aperture 18b through which a flux of light of the image to be photographed is passed. Said half mirror 9 is disposed inside said box 18 at an angle of 45 degrees relative to said master lens group 8 and said lens axis 0. The

, box 18 is provided with an opening on that portion of its peripheral wall located in the path of the reflected light 0 coming from the half mirror 9.

A photoresistor 10 constituting one arm of the Wheatstone bridge in the electric control circuit which will be described later is positioned so as to face said opening 180 by means of a bracket 19'.

On the front surface of said base plate 13 is mounted, coaxially with said lens axis 0, said diaphragm device 7 comprising a housing 20, a diaphragm operating ring 21 rotatably mounted in said housing 20, and a plurality of diaphragm blades 22 operated by said diaphragm operating ring 21. With respect to the aforesaid arrangement of the box 18 and the diaphragm device, it should be understood that the internal diameter of the box 18 is in agreement with at least the widest diaphragm aperture of the diaphragm device 7. A crank lever 24 is pivotally mounted to a shaft 23 provided on said base plate 13 so that the lever can make a pivotal movement about said shaft 23. A pin 24a is provided on one end of said crank lever so as to protrude outwardly therefrom and to engage in the slot 17a formed in said control disk 17, while a fork portion 24b formed on the other end of the crank lever engages a pin 21a formed on the diaphragm operating ring 21 so as to protrude outwardly therefrom.

It is important to note that an arrangement is provided so that during the movement of the slot 17a of the control disk 17 from one end to the other end with respect to the pin 24a of said crank lever, the diaphragm blades 22 effect a movement from their position of the widest diaphragm to their position of the narrowest diaphragm aperture.

In FIG. 4 is shown an electric control circuit comprising an electric switching circuit which comprises an electric bridge of Wheatstone type including a photoresistor 10 and two switching transistors of a p-n-p type and an n-p-n type connected between the output terminals of said electric bridge, a transistor amplifier composed of transistors connected to said switching circuit, an energizing coil L connected to the output side of said transistor amplifier, and a damping coil L connected between the junction of the common base electrode and the respective emitter electrodes of the switching transistors. The Wheatstone bridge comprises a photoresistor 10 which undergoes a change in its resistance value in response to a change in the intensity of the incident light, a variable resistor 25 to match the emulsion speed of the film used, a variable resistor 26 adapted to set the shutter speed, and a resistor 27. The junction between one end of said photoresistor 10 and said variable resistor 26 is connected by line 28 to the negative terminal of said power source E while the junction of one end of the variable resistor 25 and the resistor 27 is connected by line 29 to the positive terminal of the power source E The other terminal of the photoresistor 10 is connected to a slider 25a of the variable resistor 25, while the other end of said variable resistor 25 is connected, through a bias resistor 30, to the common base electrode of the p-n-p type and the n-p-n type switching transistors 31 and 32.

Between the respective emitter electrodes of the transistors 31 and 32 are inserted, for connection in series, two resistors 33 and 34.

In addition, another resistor or a thermistor 35 is inserted for connection in parallel to both of said resistors 33 and 34. Also, one end of the resistor 27 is connected to the joint of the resistor 33 and the thermistor 35, and one end of a variable resistor 26 is connected to the junction of the thermistor 35 and the resistor 34. Each of the collector electrodes of the switching transistors 31 and 32 is connected, through a resistor, to each of the base electrodes of transistors 36 and 37, while thermistors 38 and 39 are inserted for connection between the base electrodes and lines 28 and 29, respectively, said base electrodes being connected to each other by means of a resistor.

Furthermore, the respective emitter electrodes of said transistors 36 and 37 are connected, through resistors 40 and 41, to the lines 28 and 29, respectively. The common collector electrode of the transistors 36 and 37 is connected to the junction of the resistors 44 and 45 which are connected to the respective base electrodes of the transistors 42 and 43. An electric condenser 46 having a polarity is inserted for connection between said junction and the line 29. A resistor is inserted for connection between the junction of the resistors 44 and 45 and the common emitter electrode of the transistors 42 and 43.

The respective collector electrodes of said transistors 4-2 and 43 are connected to the lines 28 and 29, respectively. The aforedescribed energizing coil L is inserted for connection, through a first switch SW which is normally rendered open except during the period of photo graphing, between the common emitter electrode of the transistors 42 and 43 and the neutral point of said power sources E and E Also, a damping coil L which will be later described is inserted between the emitter electrodes of said transistors 31 and 32. It is important to note that said energizing coil L and said damping coil L are wound around the movable element 16 in such manner that they are not magnetically coupled to each other and also that said damping coil L is connected in such fashion that the polarity of the induced voltage generated in said coil is opposite to the polarity of the unbalanced voltage generated between the output terminals A and B of the Wheatstone bridge. A second switch SW which is normally rendered open except during the period of photographing is inserted between the collector electrode of the transistor 43 and the positive terminal of the power source E Said first switch SW and said second switch SW are actuated to close in synchronism with the release operation of the shutter release member (not shown). These switches are arranged so that the closing of these switches is eifected in such manner that said second switch SW is closed after said first switch SW is closed, while the opening of these switches is effected in such fashion that said first switch SW is rendered open after said second switch SW is rendered open.

Description will now be directed to the function of said camera equipped with the exposure control system of the present invention.

Let us assume that the resistance value of the photoresistor which is determined by the intensity of the photograph light which enters through the diaphragm device 7 and the master lens group 8 and which enters the photoresistor 10 after being reflected from the half mirror 9, is designated as R that the resistance value of the variable resistor 25 set in compliance with the emulsion speed of the film used is designated as R that the resistance value of the variable resistor 26 set synchronously with the setting operation of the shutter speed is designated as R and that the resistance value of the resistor 27 is designated as R Now, if the Wheatstone bridge is in its balanced state, or in other words, if these resistance values are in the relationship R -R =R -R then there is generated no voltage whatsoever in the output terminals A and B of said bridge even when both the first switch SW and the second switch SW are rendered into closed state by virtue of the release operation of the shutter release member, and as a consequence, neither one of the transistors 31 and 32 is actuated. Accordingly, there is no appreciable current flowing in the energizing coil L and the movable element 16 remains immovable in its present position. Assuming now that a change has occurred in the intensity of the light incident to the photoresistor 10, or in other words in the brightness of the object to be photographed, causing a change in its resistance value and that as a result the relationship between the resistance values of the respective resistors has been altered to R -R R -R there is generated an unbalanced positive voltage in the output terminal A of the Wheatstone bridge and an unbalanced negative voltage in the output terminal B, and as a consequence, the transistor 31 is rendered to a cutoff state, while the mating transistor 32 is brought into an active state. Accordingly, the transistors 36 and 42 are rendered to an off state, and the transistors 37 and 43 are put into an active state, causing a current to flow in the energizing coil L in the direction of the arrow indicated in dotted line. .On the contrary, if the relationship between the respective resistors is altered to R -R R -R an unbalanced negative voltage is generated in the output terminal A and an unbalanced positive voltage is generated in the output terminal B, causing the state of the transistors 31, 36 and 42 to be shifted to an active state, respectively, while the transistors 32, 37 and 43 are rendered to cutoff state, respectively. As a consequence, a current in the direction of the solid line is caused to flow in the energizing coil L When a current in the direction of either the dotted arrow or the solid arrow, which is the reverse of the former, flows through the energizing coil L, the movable element 16, in response to the current, rotates and changes its position either clockwise or counterclockwise about the main shaft 16a. More specifically with reference to FIG. 2, when a current in the direction of the solid line flows in the energizing coil L or in other words, when the light incident to the photoresistor 10 is intensified, the rotary shaft 16a of the electromagnetic device 16 effects clockwise rotation, causing the control disk 17 to rotate clockwise. As a consequence, the slot 17a of the control disk 17 causes, through the pin 24a, the crank lever 24 to make clockwise rotation about the pivotal shaft 23. This clockwise rotation of the crank lever 24 causes the fork portion 24b of the crank lever 24 to engage the pin 21a of the diaphragm operating ring 21 to rotate the latter ring 21 counterclockwise so as to move the diaphragm blades 22 till they assume a position of a reduced diaphragm aperture. As a natural consequence, the amount of light entering through the diaphragm aperture and into the box 18 is reduced. A portion of this reduced incident light passes straight along the lens axis 0, and through the shutter device 11, reaches the film 12 to expose the latter to said light, while the remaining portion of the incident light is reflected from the half mirror 9 and this reflected light passes through the opening to reach the photoresistor 10. The intensity of the light reaching the photoresistor 10 is gradually reduced with the gradual narrowing of the diaphragm blades. When the resistance R of the photoresistor 10 has reached, as a result of the reduced intensity of the light reaching the photoresistor 10, a value at which the Wheatstone bridge is again put into a state of equilibrium, the movable element 16 ceases its movement. On the other hand, when a current in the direction of the dotted line is passed through the energizing coil L or in other words, when the amount of the light incident to the photoresistor 10 is reduced, the movable element 16 and accordingly the diaphragm device 7 effect a movement in a direction reverse to that stated in connection with the foregoing operation, gradually increasing the amount of light reaching the photoresistor 10 until the resistance value R thereof puts the Wheatstone bridge into a state of equilibrium again at which the movable element 16 ceases its movement.

Accordingly, the amount of light reaching the film 12 is always of a constant value independently of the brightness of the object to be photographed.

The afore-discussed control means is advantageous in that it effectively eliminates, by the provision of the damping coil L having the foregoing structure and the foregoing connection pattern, the undesirable occurrence of oscillation of the movable element 16 accruing from the light amountresistance value characteristic of the photoconductive material principally constituting the photoresistor and also eliminates the inertia of said movable element 16 which is due to a sharp change in the amount of light.

A photoconductive material, in general, would not undergo a change in its resistance value immediately after a change has taken place in the amount of light incident thereto, but it requires a certain small length of time or delay in time before its resistance value effects a change in response to said change in the intensity of the incident light. The control means of the present invention works, to compensate for the foregoing defective nature of the photoconductive material, in the following manner. When there occurs a change in the resistance value of the photoresistor 10, causing the movable element 16 to rotate to change its position, said movable element 16 will cease its movement for a moment when there is established a state of equilibrium in the Wheatstone bridge, or in other words, when there .is formed a relationship R -R =R -R On account of the aforesaid delay in time, however, the resistances are placed, immediately after the cease of movement of said element 16, into a relationship R -R %R -R again rendering the Wheatstone bridge to an unbalanced state.

More concretely, when, for example, there occurs an increase in the amount of light incident to the photoresistor 10 causing the resistance value R of said resistor 10 to be reduced, the movable element 16 acts to cause the diaphragm aperture of the diaphragm means 7 to reduce its diameter or size and to thereby increase the resistance value R with a result that the Wheatstone bridge is placed again into a balanced state. However, the resistance value R of the photoresistor 10 continues to increase for a very short extended length of time after the Wheatstone bridge has already reached the equilibrium state. Such delay in the action of the photoresistor 10 causes a voltage of reverse polarity to be generated between the output terminals A and B of the Wheatstone bridge so that the movable element 16 is displaced in the direction opposite to the initial direction, to widen the diaphragm aperture. With the widening of the diaphragm aperture, the amount of light incident to the photoresistor 10 increases, resulting in that the resistance value R of the photoresistor 10 reduces again. As a consequence, the polarity of the voltage generated between the output terminals A and B of the Wheatstone bridge is again reversed, so that the movable element 16 is actuated again to reduced the size of the diaphragm aperture. The movable element 16 thus oscillates back and forth across the equilibrium point and accordingly the diaphragm aperture is not immediately fixed at an appropriate value. However, such phenomenon is effectively eliminated by the damping coil L Specifically, the movement of the movable element 16 causes an induced voltage to be produced in the damping coil L by virture of the permanent magnetic field which is resulted by the cylindrical magnet 15. Since this induced voltage acts so as to reduce the unbalanced voltage generated between the output terminals A and B of the bridge, it serves to substantially prevent the alternate on-off' actions of the transistors 31 and 32 back and forth across the equilibrium point of the bridge due to the time delay in the making of a change in the resistance of the photoresistor 10, and to thereby bring the movable element 16 to a halt immediately upon the bridge being rendered to a balanced state.

Furthermore, since the magnitude of the induced voltage generated in the damping coil L is proportionate to the displacement velocity of the movable element 16, the latter would not change its value R of the photoresistor 10, but instead, the movable element 16 can be displaced always at a substantially constant velocity.

Let us now, designate the resistance values of the biasing resistor 30, and the resistors 33 and 34 as r r and r respectively, and the maximum unbalanced voltage generated between the output terminals A and B of the bridge as e and the induced voltage generated in the damping coil L when the movable element 16 has made a displacement at the highest possible speed as e It will be easily understood by those skilled in the art that a most effective damping action of the damping coil L is obtained by selecting the values r 1' and r so as to satisfy the following relationship:

Also, in the aforesaid control means, it sometimes occurs that neither of the transistors 31 and 32 is actuated depending upon the magnitude of the unbalanced voltage generated between the output terminals of the bridge resulting principally from the V I characteristic of the transistors 31 and 32. More specifically, when the resistance value of the photoresistor 10 effects a very limited change, the magnitude of the unbalanced voltage generated between the output terminals A and B is accordingly very small. In such case, neither of the transistors 31 and 32 is actuated. Since in the arrangement of the present invention, a resistor or a thermistor 35 is inserted for connection between one of the output terminals, namely, between B and B of the bridge to eliminate the aforesaid inconvenience, it is possible to prevent the dynamic sensi-' tivity of said transistors 31 and 32 from diminishing owing to the influence of the voltage V between the base electrode and the emitter electrode of these transistors, and it is also possible to actuate either the transistor 31 or the transistor 32 even when an unbalanced voltage of a slight degree is generated between the output terminals A and B of the bridge.

By the use of said thermistor in the electric control circuit, it is also possible to prevent a change in the dynamic sensitivity of the transistors 31 and 32 due to a change in the ambient temperature from taking place.

Furthermore, in the control means described above, the current which flows in the energizing coil L tends to contain an alternating component of a relatively high frequency due mainly to such factors as the structure of the electromagnetic device 6 per se, or due to the coupling relation between the electric control circuit and the electromagnetic device 6, resulting in an inconvenience that the movable element 16 Will make a stepwise movement during its displacement motion.

Since, according to the present invention, there is inserted an electric condenser 46 between line 29 and the junction of the resistors 44 and 45 connected to the base electrodes of the transistors 42 and 43, the aforesaid alternating component is positively bypassed so that the alternating component can be removed from the current flowing through the energizing coil L As a consequence, the movable element 16 can always effect a smooth and continuous motion of displacement.

In this instance, said condenser 46 may be inserted between line 28 and the junction of the resistors 44 and 45 connected to the base electrodes of the transistors 42 and 43, and furthermore, an identical effect can be obtained by inserting, as are indicated by the dotted lines in FIG. 4, a condenser between the collector electrode of the transistor 31 and the junction of the resistor 10 and the variable resistor 26, and by inserting still another one between the collector electrode of the transistor 32 and the junction of the variable resistor 25 and the resistor 27, respectively.

As has been discussed, the diaphragm device 7 is controlled, according to the present invention, smoothly and positively in compliance with the brightness of the object to be photographed, after taking the emulsion speed of the film used and the shutter speed into consideration, and accordingly the face of the film is exposed always to a constant and adequate amount of light.

3 max.

Since an arrangement is provided that the opening of the switches SW and SW is eifected in such manner that the first switch SW is opened after the second switch SW has been opened as has been described, the circuit comprising the transistors 32, 37 and 43 is rendered to a cutoff state first. The circuit including the transistors 31, 36 and 42, at this moment, is still in the active state regardless of the resistance value of the photoresistor 10. As a consequence, when the switch SW is opened, a current in the direction of the solid line flows in the energizing coil L causing the diaphragm blades 22 to quickly move toward the position of the narrowest diaphragm aperture or the position of perfect closure. As a consequence, at each photographing operation, the diaphragm blades 22 are started always at the position of the narrowest diaphragm aperture or the position of perfect closure. Said second switch SW may be inserted between the collector of the transistor 42 and the negative terminal of the power source E In such arrangement, the diaphragm blades 22 will be started always at the position of the widest diaphragm aperture.

The exposure control means of the present invention which is applied to a cinecamera has been described. It is needless to say that the exposure control means of the present invention may be applied also to a still camera. It should be understood clearly by those skilled in the art that various modifications, alterations and improvements of the structure and the functions of the minor parts of the exposure control means of the present invention which has been described with reference to the accompanying drawings may be made without departing from the spirit of the present invention.

We claim:

1. An exposure control system for photographic cameras with an objective lens system, comprising: a diaphragm aperture; a Wheatstone-type bridge having a photoresistor in one of its branches; means to direct a part of light passing through said aperture onto said photoresistor; a diaphragm control means responsive to an unbalance voltage generated between the output terminals of said bridge by light directed on said photoresistor to adjust said diaphragm aperture to cause said bridge to regain a balanced state; said diaphragm control means comprising an electromagnetic device including at least one permanent magnet, a movable element influenced by the magnetic field of said magnet, a diaphragm adjusting means, coupled to said electromagnetic device, and an electric control circuit for actuating said electromagnetic device; said electric control circuit comprising said Wheatstone-type bridge, switching transistors of p-n-p type and n-p-n type connected between the output terminals of said bridge and having a common base electrode, and at least two amplifying transistors, one for and connected with each of the collector electrodes of said switching transistors, said amplifying transistors having a common emitter electrode; said movable element wound with an energizing coil connected between the common emitter electrode of said amplifying transistors and power sources and also wound with a damping coil connected between the output points of connection of said bridge in such manner that the polarity of the voltage is reverse to the polarity of the unbalance voltage generated between the output terminals of said bridge.

2. An exposure control system for photographic cameras according to claim 1, further comprising: a biasing resistor connected between one of the output points of connection of said bridge and the common base electrode of said switching transistors; two resistors of an equal resistance value connected in series between the emitter electrodes of said switching transistors; said damping coil member connected between said common base electrode of said switching transistors and the point of connection of said two resistors in series; means to provide the selection of the ratio of the resistance value of either one of said two resistors in series to the total resistance value of said biasing resistor substantially equal to the ratio of the voltage induced in said damping coil member, when said movable element effects displacement at the maximum permissible displacement velocity, to the maximum unbalanced voltage produced between the output points of said bridge.

3. An exposure control system for photographic cameras according to claim 1 further comprising a resistor connected between the respective emitter electrodes of said switching transistors, thereby to prevent a reduction in the dynamic sensitivity of said switching transistors.

4. An exposure control system as claimed in claim 3, said resistor inserted between the emitter electrodes being a thermistor.

5. An exposure control system for photographic cameras according to claim 1 further comprising: two biasing resistors connected in series between the base electrodes of said two amplifying transistors; and an electric condenser connected between the series junction of said two biasing resistors and the collector electrode of either one of said two amplifying transistors.

6. An exposure control system for photographic cameras according to claim 1 further comprising electric condensers connected between the respective collector elctrodes of said switching transistors and the input points of connection of said bridge.

7. An exposure control system for photographic cameras according to claim 1 further comprising a first switch means connected to said energizing coil and a second switch means connected to the collector electrode of either one of said amplifying transistors, said second switch means closing when said diaphragm control means starts its action after said first switch means has been closed; said first switch means opening when said diaphragm control means completes its action after said second switch means has been opened.

8. An exposure control system according to claim 1, said Wheatstone bridge having a shutter speed resistor and means to vary it in compliance with the shutter speed.

9. An exposure control system according to claim 1, said Wheatstone bridge having an emulsion speed resistor and means to vary it in compliance with the emulsion speed of the film used.

References Cited UNITED STATES PATENTS 3,197,783 7/1965 Ahn et al 352141XR 3,230,847 1/1966 Gregory et al. 352-141 XR FOREIGN PATENTS 1,333,253 6/1963 France. 1,047,379 9/ 1964 Great Britain.

NORTON ANSHER, Primary Examiner JOSEPH F. PETERS, JR., Assistant Examiner U.S. Cl. X.R. 

